Electron beam generators of different designs have been used for many decades for the implementation of industrial, high-speed PVD methods. Electron-beam vaporizers provide the fastest, industrial-scale, proven coating speeds while delivering excellent uniformity and purity of the deposited layer, and this applies to both reactive and ferromagnetic coating materials as well as to coating materials with a high melting temperature. These characteristics result from a high power flux density, which is adjustable without inertia loss through magnetic focusing and beam control, as well as from the direct heating of a vapor-releasing surface, whereby the crucible needed to hold the coating material can be cooled and thus no contamination of the coating will result.
The electron sources used for industrial PVD processes are presently based exclusively on cathodes heated to a high operating temperature, wherein the generation of free electrons is based on the thermionic effect. The functioning principle of these “thermionic cathodes” means that traditional electron emitters have a complicated design, their power supplies are relatively expensive, and certain embodiments can cover only a highly limited range of technological applications.
One widely used vapor source for vaporization by electron beam is the transversal electron beam generator, wherein the beam generation, the magnetic 270° beam deflection and a crucible with vaporization material are usually integrated in one compact functional unit. These sources are relatively low in cost, but they are limited with respect to their maximum radiation power (approximately 20 kW) and their acceleration voltage (approximately 20 kV). As a result, these sources also are limited with respect to the producible vaporization rate. In addition, the actual radiation source (cathode and heating) is located at the pressure level of a respective coating chamber and is thus exposed directly to the vapors and gases found therein. Consequently, the pressure in the coating chamber must be kept to low values by a correspondingly generous dimensioning of the vacuum pumps in order to prevent instabilities during operation of the electron source. In high-speed deposition of dielectric compounds which require a reactive process control, that is, the setting of a relatively high partial pressure (0.1 to 1 Pa) of reactive gases in the vacuum chamber to ensure the necessary stoichiometry, transversal electron beam generators have not ultimately proven useful in spite of numerous improvements in their design and circuitry. In particular, transversal electron beam generators have not proven useful due to their great tendency toward high voltage arcing, which is unacceptable under these process conditions.
A technologically higher-performance beam generator for vaporizing with electron beams are so-called axial electron beam sources, which are designed for vaporization methods with beam powers up to 300 kW and acceleration voltages up to 60 kV. The cathode chamber of these beam sources is evacuated through apertures with small, usually circular openings for the passage of the beam. The apertures function as a flow resistance to the vacuum. The cathode chamber is separate from the process chamber and is separately evacuated with additional high-vacuum pumps. In the present embodiment, this evacuation is accomplished by means of turbomolecular pumps. Thus the vaporization process can also be operated even at higher pressures, and in particular, also with a large proportion of reactive gases in the coating chamber. In addition, greater coating rates can be attained without any loss of stability. However, systems of this kind are quite expensive with respect to their capital investment costs, and thus for economic reasons can be used profitably only in a narrow range of applications.
In order to overcome this limitation, various cold cathode beam sources with plasma anodes have been developed, wherein the liberation of electrons is not based on the thermionic effect, but rather on the firing of ions from a large-area metal electrode. A high-voltage glow discharge maintained in the beam source produces ions and accelerates them to the cathode. The electrons ballistically transferred from the solid body into the vacuum are accelerated in the fall to the cathode through the plasma, and due to suitable electrode contours, are shaped into a homocentric beam which can be focused by conventional electron-optical subassemblies and deflected to the evaporator.
Whereas thermionic emitters require a high vacuum of better than 10−3 Pa in the cathode chamber, the operating pressure of the cold cathode is in the range of 2 to 5 Pa. Therefore a differential evacuation of the beam source up to a pressure of about 1 Pa in the coating chamber can be omitted. The control of beam power is effected here by variation of the plasma density in the cathode chamber by a fast gas flow control. Instead of the usual multi-conductor high-voltage power supply, a unipolar high-voltage supply suffices, and the high-voltage power supply does not require any additional floating power supply at high electric potential. As one important financial benefit, it is emphasized that systems designed on the basis of cold cathode emitters—comprising the beam source including its power supply and control components—can be produced at significantly lower costs compared to conventional axial emitter systems.
The described cold cathode axial emitters thus feature many advantages over conventional thermionic emitters, yet still exhibit some shortcomings in certain technical parameters or for particular applications. The competing requirements for reduction in field intensity for the cathode (requires the greatest possible electrode spacing) and reliable dark field shielding (requires the smallest possible electrode spacing) make it increasingly difficult for plasma-based beam sources with increasing operating pressure to maintain the high acceleration voltages in a stable manner over a long term. Previously the operating voltages of around 30 kV prevailing in the high-power range with cold cathode emitters have been sufficient for high-vacuum coating processes, such as, for example, metal coating (0.001 to 0.01 Pa). But voltages in the range of 40 to 60 kV would be more expedient for reactive, high-speed coating processes with typically far greater pressure in the process chamber (0.1 to 1 Pa), due to the better energy transport capacity of the beam.
One important disadvantage of plasma-based electron beam generators is the frequent transition of the glow discharge into an arc discharge. This electric arc in most cases is not extinguished automatically. Due to a number of improvements in technical operation and in source design, the arc rate can be reduced to a value which is compatible at least for selected processes, and some improvements require a considerable, additional outlay of financial resources or for process control. Nonetheless, the arc rates achieved are still too high for many processes.
In order to benefit from the economic advantages of plasma-based beam generators, without the undesirable restrictions on their range of potential applications, a power supply with fast arc handling is required.
The process of arc handling is similar in most power supplies and largely follows the following outline: an arc is detected (arc detection time) and if necessary, a short time (feed time) is intentionally allowed, in order to burn off any possible flakes from the cathode. Next, the energy supply is interrupted for a certain recovery time by switching off the power supply. During this time, the arc is extinguished and the supply voltage can be switched on again. After switch-on, the power supply needs a little time to build up to operating voltage again. The sum of the individual time periods results in the latency time for the process, since the beam generation is interrupted in these phases. Some power supplies additionally limit the current until the arc is extinguished. Power limiting and fast extinction of the arc are used to protect the beam source and ensure its long-term stability.
In selected processes—meaning here in particular the field of high-speed coating, for example of packaging foil—a latency time in the range of microseconds is needed for a reliable prevention of arc-induced defects in the product, such as for example, insufficient coating thickness. Power supplies with fast arc handling in the millisecond range can only be employed in a financially reasonable manner for a limited power range, and are based on medium-frequency technology. Thus these power supplies are too expensive for power outputs greater than 60 kW, and are generally replaced by controlled thyristor controllers. They operate on the existing mains frequency of 50 Hz. The power supply in the case of thyristor controllers can only be switched off by an ignition lock of the thyristors, since thyristors can only be switched on, but cannot be switched off. Therefore the energy supply can only be interrupted after the natural zero transit of the primary alternating voltage. In a 6-pulse rectifier, in the worst case the interruption can occur only after 3.33 ms. In addition, with a thyristor controller, the output capacitance is several times greater in comparison to medium frequency technology because the undulating output voltage requires a greater smoothing. Therefore, in the event of an arc, the entire energy of this capacitance must be discharged into the arc, which can result in an extreme power surge and damage to the power supply.
No solutions are known for operation of the electron beam sources names above, with power outputs greater than 60 kW with a latency time of arc suppression in the range of approximately 100 μs.